I know that this isn't really possible, but I want to discuss the possibilities that a Homebrew community would open up with the HD (much like what the Palm Pre has)
The first and foremost thing to me would be themes/fonts, along with a couple of browser tweaks and more customization in terms of menu layout and the like.
Additionally, I'm quite aware of the limitations of the CE platform that the HD's firmware is based upon in contrast to the far-more-open Linux base webOS has.